The present inventors have proposed a variety of cycloolefin random copolymers formed from ethylene and specific cycloolefins, as described in, for example, Japanese Patent Laid-Open Publication No. 168708/1985 and Japanese Patent Applications No. 220550/1984, No. 236828/1984, No. 236829/1984 and No. 242336/1984. The cycloolefin random copolymers described in those publication and applications have high transparency and are excellent in various properties such as heat resistance, thermal aging resistance, chemical resistance, solvent resistance, dielectric properties and mechanical properties. Further, such cycloolefin random copolymers have good adhesion to substrates of various materials, though they are a kind of polyolefins. Accordingly, the cycloolefin random copolymer can be used, for example, in a field of optical materials such as optical memory discs and optical fibers, and a field of transparent container materials and packaging materials such as transparent films and transparent containers.
The cycloolefin random copolymer is conventionally used in the form of pellets for the production of molded articles. In the conventional process for preparing pellets, as shown in FIG. 2, a large number of steps are required prior to pelletizing the cycloolefin random copolymer, such as steps of polymerization, deashing, filtration, precipitation-filtration, extraction-filtration, drying, and addition of stabilizer. After these steps, the cycloolefin random copolymer is fed to an extruder, melted in the extruder, and pelletized. That is, the cycloolefin random copolymer is generally prepared by copolymerizing ethylene and a cycloolefin in a hydrocarbon solvent in the presence of a catalyst, so that steps for removing the catalyst and the solvent are required prior to the pelletizing step. Further, the unreacted cycloolefin, a low molecular weight cycloolefin random copolymer, etc., which might cause mold staining in the molding process of the resultant pellets, still remain in the hydrocarbon solvent, so that they also must be removed prior to the pelletizing step.
For preparing the cycloolefin random copolymer pellets, therefore, the catalyst, the reaction solvent, the unreacted cycloolefin and the low molecular weight cycloolefin random copolymer are removed through the above-mentioned many steps.
The cycloolefin random copolymer must go through a large number of steps prior to the pelletizing step as described above, and hence the cost for preparing the pellets becomes relatively high. Moreover, as a result of studies by the present inventors, they have found that the optical properties of the cycloolefin random copolymer, such as transparency, tends to lower every time the copolymer goes through each step after the polymerization step, though the lowering level is very low.